Transcript Routing Strategies

Routing Strategies

Fixed Routing
A
route is selected for each source-destination
pair of nodes in the network
 No
difference between routing for datagrams
and virtual circuits
 Simplicity,
but lack of flexibility
 Refinement:
supply the nodes with an alternate
next node for each destination
Routing Strategies (cont)

Flooding
A
packet is sent by a source node to every one of
its neighbors
 At
each node, an incoming packet is retx on all
outgoing link except for the link on which it arrived
 Hop
count field deals with duplicate copies of a pkt
 Properties

All possible routes btw source and destination are tried

At least one copy of the packet to arrive at the destination
will have a minimum-hop route

All nodes connected to the source node are visited
Routing Strategies (cont)

Random Routing
 Selects
only one outgoing path for retx of an
incoming packet
 Assign
a probability to each outgoing link and
to select the link based on that probability

Adaptive Routing
 Routing
decisions that are made change as
conditions on the network change

Failure

Congestion
Routing Strategies (cont)

Adaptive Routing
 State
of the network must be exchanged
among the nodes
 Routing
decision is more complex
 Introduces
traffic of state information to the
network
 Reacting
too quickly will cause congestionproducing oscillation
 If
it reacts too slowly, the strategy will be
irrelevant
Shortest Path Algorithm

Dijkstra’s Algorithm

Bellman-Ford Algorithm
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• Reduced graph
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Dijkstra’s Algorithm
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1
D2 = 2
4
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D3 = 5
6
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D4 = 1
D4 = 1
1
3 D3 = 3
D2 = 2
4
D4 = 1
D3 = 4
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D5 = 2
M = { 1, 4 }
M={1}
2
D2 = 2
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D5 = 2
M = { 1, 2, 4, 5 }
D6 = 4
2
1
D2 = 2
4
D4 = 1
3
D3 = 4
5
D5 = 2
M = { 1, 2, 4 }
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Dijkstra’s Algorithm (cont)
Node
D
M
1
1, 4
1, 4, 2
1, 4, 2, 5
1, 4, 2, 5, 3
1, 4, 2, 5, 3, 6
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
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
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
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Dijkstra’s Algorithm (cont)

w(i, j) = link cost, L(n) = path cost from node s to n

1. [Initialization]
T
= {s}
 L(n)

= w (s, n) for n ≠ s
2. [Get next node]
x  T such that L(x) = min L(j)
jT
 Add x to T
 Find

3. [Update Least-Cost Paths]
 L(n)
 Go
= min [ L(n), L(x)+w(x, n) ] for all n  T
to step 2
Bellman-Ford Algorithm
D(2)3 = 4
2
1
3
D(1)2 = 2
4
D(1)3 = 5
2
1
6
3
D(2)2 = 2
5
4
D(1)4 = 1
5
D(2)4 = 1
h=1
h=2
D(3)3 = 3
2
1
3
D(3)2 = 2
4
6
5
D(3)4 = 1
h=3
D(3)5 = 2
6
D(3)6 = 4
D(2)5 = 2
D(2)6 = 10
Bellman-Ford Algorithm (cont)
Node
D
h
Source = 1
0
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2
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


2
5
1




2
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3
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1
1
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10
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Bellman-Ford Algorithm (cont)

Lh(n) = path cost from s to n w/ no more than h links

1. [Initialization]

 L0(n)
= ∞, for all n ≠ s
 Lh(s)
= 0, for all h
j
s
2. [Update]
 For
each successive h ≥ 0
 For
each n ≠ s, compute
 Lh+1(n)
link
= min [ Lh(j) + w(j, n) ]
j
<= h links
n
Comparisons
L(n) = min [ L(n), L(x)+w(x, n) ]
Lh(x, D)
x
S
…
D
S
x
……
x
Dijkstra’s
(Link State)
Bellman-Ford
(Distance Vector)
D
Routing in ARPANET

First generation(RIP), 1969
 Adaptive
 Use
Routing is adopted
Bellman-Ford algorithm
 Estimated
link delay is simply the queue length
for that link
 Every
128ms, each node exchanges its delay
vector(routing table) with all its neighbors
 Information
about a change in network condition
would gradually ripple through the network
Routing in ARPANET (cont)
 Each
node i maintains

di j = current estimate of min delay from i to j

si j = next node in the current min-delay route from i to j
 Node
k updates its vectors as follows

dk j = Min [ lk i + di j ]
iA

sk j = i
k
i
j
using i that minimizes the expression above
where
A = set of neighbor nodes for k
lk i = current estimate of delay from k to i
Routing in ARPANET (cont)

Major shortcomings of RIP
 It
did not consider line speed, merely queue
length. Higher capacity links were not given
the favored status
 Queue
 The
length is an artificial measure of delay
algorithm was not very accurate. It
responded slowly to congestion and delay
increases.
Routing in ARPANET (cont)

Second generation, 1979
 OSPF:
Open Shortest Path First protocol
 Link-state
 The
routing protocol
delay is measured directly
 Every
10 seconds, the node computes the
average delay on each outgoing link
 Information
of changes in delay is sent to all
others nodes using flooding
 Using
Dijkstra’s algorithm
Routing in ARPANET (cont)
Routing in ARPANET (cont)

Third generation, 1987
 Problem

The correlation between the reported values (delay)
and those actually experienced after rerouting
 Conclusion

Under heavy load, the goal of routing should be to
give the average route a good path instead of
attempting to give all routes the best path
 Solution

Also consider the average utilization of links

Revised cost function: delay-based metric under
light loads, capacity-based metric under heavy loads
Calculate Link Costs
1.
Measure the avg. delay over the last 10 sec
2.
Using the single-server queuing model, the
measured delay is transformed into an
estimate of link utilization
3.
Average the link utilization with the
previous estimate of utilization
4.
The link cost is set as a function of average
utilization
ARPANET Delay Metrics (3rd)
Theoretical
queueing delay
Delay (hops)
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4
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2
1
Metric for
satellite link
Metric for
terrestrial link
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Estimated utilization